The document “System description, Wireless Power Transfer, Volume I: Low Power, Part 1: Interface Definition, Version 1.0 Jul. 2010, published by the Wireless Power Consortium” available via http://www.wirelesspowerconsortium.com/ downloads/wireless-power-specification-part-1.html, also called the Qi wireless power specification describes wireless transmission of power.
To prepare and control the power transfer between a power transmitter and a power receiver in such a wireless, inductive power transfer system, the power receiver communicates information to the power transmitter. For example, the power receiver may communicate a data packet indicating the received power, e.g. the rectified power.
A problem is that metal objects, positioned at the surface of the power transmitter, can reach an undesired high temperature (higher than 65° C.) due to eddy currents caused by the magnetic field generated by the power transmitter. This is an unwanted situation. Skin burning and plastic melting could result from this heating.